Support of multiturn mode for MX motors for suspended pypot creatures

[ThotAlion]

For suspended creatures (hung by a string) we would need to take advantage of the multi-turn mode of the MX motors so that we can stick a pulley on the motor rolling a string like a winch.
More generally : there are three modes : joint, wheel and multiturn.
For the joint mode, all is OK
For the wheel mode, set angle limit registers to 0 (not possible in the json since angle limits are converted)
For joint mode, set angle limits to the one you want
For multi-turn mode, set angle limits to 4095.
For multi turn mode, we should add two others properties : multi-turn offset and resolution divider.

I wanted to add it by myself but it is not so simple.

[pierre-rouanet]

Hi @ThotAlion,

I never had the opportunity to play with the multi-turn mode and I don't really have MX motors around.

But let me know where you found the difficulties to add those features and I'll try to help. This would be a cool enhancement for pypot. Do not hesitate to open a WIP pull request where we could directly work and discuss this.

[montequie]

Hi, was it ever implemented?

[pierre-rouanet]

No it's still an open issue. A few pointers to help out anyone willing to work on this:

• As @ThotAlion pointed out, to enable multi turn mode you need to set a specific angle limit. This can be done here:

  pypot/pypot/dynamixel/io/io.py

  Lines 20 to 53 in 69814e9

  	def get_control_mode(self, ids):
  	""" Gets the mode ('joint' or 'wheel') for the specified motors. """
  	to_get_ids = [id for id in ids if id not in self._known_mode]
  	limits = self.get_angle_limit(to_get_ids, convert=False)
  	modes = ['wheel' if limit == (0, 0) else 'joint' for limit in limits]
  	self._known_mode.update(zip(to_get_ids, modes))
  	return tuple(self._known_mode[id] for id in ids)
  	def set_wheel_mode(self, ids):
  	""" Sets the specified motors to wheel mode. """
  	self.set_control_mode(dict(zip(ids, itertools.repeat('wheel'))))
  	def set_joint_mode(self, ids):
  	""" Sets the specified motors to joint mode. """
  	self.set_control_mode(dict(zip(ids, itertools.repeat('joint'))))
  	def set_control_mode(self, mode_for_id):
  	models = []
  	for m in self.get_model(list(mode_for_id.keys())):
  	if m.startswith('MX'):
  	models += ['MX']
  	elif m.startswith('SR'):
  	models += ['SR']
  	else:
  	models += ['*']
  	pos_max = [conv.position_range[m][0] for m in models]
  	limits = ((0, 0) if mode == 'wheel' else (0, pos_max[i] - 1)
  	for i, mode in enumerate(mode_for_id.itervalues()))
  	self._set_angle_limit(dict(zip(mode_for_id.keys(), limits)), convert=False)
  	self._known_mode.update(mode_for_id.items())

  and here:

  pypot/pypot/dynamixel/conversion.py

  Lines 327 to 330 in 69814e9

  	control_modes = {
  	1: 'wheel',
  	2: 'joint',
  	}

• Adding the two extra properties (offset and resolution divider) can be done at the end of the io.py file: https://github.com/poppy-project/pypot/blob/master/pypot/dynamixel/io/io.py by adding a new control with the correct spec from robotis support website. Such as in here:

  pypot/pypot/dynamixel/io/io.py

  Lines 121 to 126 in 69814e9

  	_add_control('drive mode',
  	address=0x0A, length=1,
  	access=_DxlAccess.readwrite,
  	models=('MX-106', ),
  	dxl_to_si=conv.dxl_to_drive_mode,
  	si_to_dxl=conv.drive_mode_to_dxl)

This should be a good start!